GaN based compound semiconductor such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (AlGaN) is drawing attention as a material for blue emission light-emitting diode (LED) or laser diode (LD). Further, development for application of GaN based compound semiconductor in elements for electronic devices is taken up by employing such a merit that it is excellent in heat resistance and environment resistance.
As there is difficulty in bulk crystal growth of GaN based compound semiconductor, it is hard to obtain a bulk substrate made of GaN capable of bearing the load of practical use. As of now, a substrate that is put to wide practice use for growing GaN thereon is just sapphire substrate, and such approach is generally employed that GaN is grown epitaxially on a single crystalline sapphire substrate by means of such growth technique as organometallic vapor phase epitaxyl (MOVPE).
Meanwhile, there is difference in lattice constant between the sapphire substrate and GaN; therefore, at such a case that GaN is grown directly on a sapphire substrate, it is impossible to grow a single crystalline film thereof. Hence, worked out (JP-A-63-188983) was such a process where a buffer layer of AlN or GaN was grown at a low temperature in advance on a sapphire substrate, which resulted in relaxation of the strain of lattice with use of the low-temperature-grown buffer layer, and then GaN was successfully growth thereon. Use of said low-temperature-grown nitride layer as a buffer layer has made single-crystalline epitaxial growth of GaN. Even in this method, however, there is no help for the lattice mismatch between the substrate and GaN crystal, and thus the GaN obtained therein has a number of defects. There is such anticipation that the defects will play new hurdles on the road to production of GaN based LD. Further, in recent years, as for method of reducing the density of the defects which are generated owing to the difference in lattice constant between sapphire and GaN, there have been reported growth techniques, such as ELO [Appl. Phys. Lett. 71 (18) 2638 (1997)], FIELO (Facet-initiated Epitaxial Lateral Over-growth) [Jpn. J. Appl. Phys. 38, Part 2, No. 2B, L184 (1999)] and pendeo epitaxy [MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999)]. As a result, GaN epitaxial wafers having significantly improved crystal quality have come to be obtained.